<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
  .parent{
      border:1px solid red;   
   }
  .child1 {
    border: 1px solid skyblue;
    margin-bottom: 30px;
    /* display: inline-block; */
    /* overflow: hidden; */
  }
  .child2 {
    border: 1px solid green;
    margin-top: 30px;
    /* display: inline-block; */
  }

  </style>
</head>
<body>
  <div class="parent">
     <div class="child1">第一个</div>
     <div class="child2">第二个</div>
  </div>
</body>


</html>